Coal bed methane (CBM) is a natural gas contained in coal seams. Methane gas is formed during coalification, the transformation of plant material into coal. Also called coal seam gas, CBM is usually not found in the atmosphere until it is released during coal mining. The gas released during mining is called coal mine methane (CMM).

According to the United States Environmental Protection Agency, 88 billion cubic feet of CMM can provide electricity to more than 1.2 million homes for a year. CBM is another source of methane, which is also used to heat buildings.

At the start of the twenty-first century, CBM drilling was underway in states such as New Mexico, Colorado, Wyoming, and Montana. The gas is often located in aquifers, porous rock that contains water. Gas is obtained by pumping thousands of gallons of water out of a CBM well.

This process could negatively affect the environment because groundwater frequently contains salt and other minerals. Removed water is reinjected into the ground in New Mexico and Colorado. Reinjection is not always possible, which brings more environmental concerns. In 2002, Montana residents worried that groundwater released into rivers and streams could hurt crops, wildlife, and drinking water.